Fixed: bug in Formula::Normalize(); improved Formula::Flatten().

* Formula::Normalize() was missing a negation: normalization of ~c
  should be ~c when c is no unit clause, but it was just c.
* Formula::Flatten() now also leads to clausal form in a negated
  context provided the original clause is a unit clause: it now
  adds a double negation.
* Improved error reporting in Parser.